Cartridge heaters for producing heat in devices, such as plastic molding machines, are well known. Such heaters include an electrical heating element suitably enclosed in a cylindrical or polygonal metal sheath or casing that is usually made of a suitable steel alloy. The heaters are adapted to be inserted into bore holes for producing a close fit to produce maximum heat transfer from the cartridge heater to the transfer device. Because of the dose fitting bore holes in which the heaters are inserted after multiple cycles of high temperature operations, it is common that the cartridge heater will be difficult, if at all possible, to remove from the bore hole. In effect, the cartridge heater seizes in the bore hole after operation. Thereafter, removal of the cartridge heater may cause damage to the heater as well as to the device in which it is used.
Heretofore, in order to facilitate removal of such cartridge heaters from bore holes, various lubricants have been applied to the casings or sheaths of the heaters prior to insertion into the bore holes. For example, it is known to have dipped cartridge heaters into milk of magnesia in order to facilitate removal after operation.
It has also been known to apply bonded solid lubricants, such as disclosed in U.S. Pat. No. 5,136,143. The preferred solid lubricant in this patent is molybdenum disulfide. This patent also suggests that other solid lubricants could be used, which include fluropolymers, ceramic materials, oxides, and mineral powders such as graphite. The patent describes the solid lubricant to require a temperature of greater than 400° F. during use before the lubricant is fully cured and describes the lubricant to have degraded into a flaky or powdery form that separates from the sheath when the heater is removed from the bore hole after use.
Other types of coatings have been suggested for operating electrical resistance heating elements in other environments.
The problem of prior art coatings for cartridge heaters is that the coatings must be cured at a very high temperature and that the coating can degrade and flake off upon removal of the heater from a device.